Cartridge type electrical fuses having axial leads have been long known in the fuse art. The fuse element in such a fuse is typically a fusible wire centrally supported within a cylindrical sleeve forming a casing for the fuse. To insure reliable fusing it is essential that the fuse wire must not touch the interior wall of the sleeve along its length, hence, the ends of the fuse wire are supported in such a manner as to prevent such contact. External lead carrying end caps having solder therein are used to capture the fuse wire ends folded over the outside of the sleeve ends. Final mechanical assembly consists of press fitting the end caps over the folded-over ends of the fuse wire followed by momentary heating of the solder to obtain good electrical connection between the fuse wire and the end caps.
Since the fuse casing formed by the sleeve must form an insulated body, typically made of ceramic or glass, which cannot be easily solder bonded, the only substantial opposition to the separation of the end caps from the sleeve is derived from the pressure fitting of the end caps over the outer surface of the sleeve. Thus, such fuse structures are generally weak in tension, and are prone to mechanical failure on a pull test applied to the end leads. Such structures are prone to humidity induced corrosion problems because of the exposed metal end caps and the lack of any hermetic sealing.
Another problem with cartridge fuses is that a certain percentage of the fuses are defective due to failure to obtain good electrical connections between the fuse link and the end caps during the soldering process. These fuses must, of course, be discarded or reworked.
One prior art solution to the above problems comprises the application of a length of heat-shrinkable plastic tubing over the sleeve and end caps, the tubing overlapping, although loosely, the inner end of the leads and extending outwardly from the end caps. The heat shrunk tubing provides some improvement in fuse strength and provides a moderately good sealing for the fuse interior. A disadvantage of this construction is that the cap ends are exposed to the external ambient conditions, owing to the fact that the limited shrinkage capability of the tubing prevents a desired end cap sealing engagement of the heat shrunk tubing with the leads. Thus, it is necessary to plate the end caps to secure adequate corrosion resistance for these elements. The resulting structure is still not adequately strong, in that a moderate pull on the leads can still shift the end caps to break the fuse wire. Also, this type of fuse construction has no effect on the fuses which are defective due to improper solder connections.
Another prior art fuse which attempts to solve some of the above problems is comprised of a cartridge fuse having an insulating sleeve, end caps, and the adjacent portions of the power leads extending therefrom, coated with a high bond strength insulating material, as, for example, an epoxy material. Such a fuse is shown in U.S. Pat. No. 4,385,281. The bonded insulating coating covers and anchors the end caps to the casing-forming sleeve. However, problems with fuses defective to poor solder connections are still present and epoxy, which is a thermo-setting plastic material, will often crack or shatter at high temperatures.